travel and rhum

I can see in NPS and other sites that uphill hikers have right of way if a trail is only one person wide. I'm curious though how folks interpret that if it's technically flat — are the people outbound considered "uphill"? If it's a trail with lots of up and down, I'm assuming it swaps with the terrain? Asking mostly because I've been on a weeklong hiking trip with a lot of popular trails and I swear I've yielded 90% of the time. I mostly don't mind but by the end it adds a lot of time.